1. Field of the Invention
A GFI having a normally closed power switch that upon the occurrence of a fault is opened and magnetically latched open, being resettable by discharge of a charged condenser across a magnetic unlatching coil.
2. Description of the Prior Art
The purpose of a GFI is to protect people from an electric shock that could cause injury or even death. Usually an electric shock is caused by a breakdown in a tool or appliance of insulation between an electrically live element and a conductive housing that is in contact with a person using the tool or appliance. In a three-wire circuit, i.e. one having a hot line, a neutral line and a grounding conductor, such a breakdown in insulation should enable the fault current to flow back to the source through the grounding conductor, thus eliminating the potential hazard. In practice, however, the grounding path may not be efficient, due to poor contacts, terminations and other factors that give rise to resistance in the grounding path. The only protection normally provided in a circuit is a fuse or circuit breaker both of which require an excess of the full rated current of the circuit to activate the protective device in order to de-energize the circuit. For example, in a 15 amp. circuit, 201/2 amps. are necessary for the circuit breaker or fuse to open. Indeed, Underwriters' Laboratories (U.L.) allots one hour for the protective device to open the circuit at this current.
It has been established that less than 0.1 amps. at 110 volts AC may engender fibrillation of the heart followed by death. Quite apparently, fuses and circuit breakers are designed to protect tools, appliances and supply wiring and afford absolutely no protection against these very small fault currents that may occur because of an insulation breakdown and the touching by a person of a part of a tool or appliance that should be electrically dead but is not. It also is apparent that in a two-wire circuit emplying two-wire plugs and in which there is no grounding conductor there is absolutely no protection to a person due to a ground fault.
GFI's are designed to sense a fault current, that is to say, a current from a live wire to ground which does not flow through the return line, which fault current is of a very low order of magnitude such as to be non-harmful to a person. A fault current that will actuate a GFI to open a circuit is as little as 5 milliamperes over a very short span of time. Preferably, this time is as little as one and one-half cycles of a conventional 60 cycle per second AC power supply, i.e. as little as 1/40th of a second. Such protection is suitable to eliminate the possibility of serious electric shock injury and death.
GFI's are presently commercially available. Their general nature is outlined in an article entitled "Barring Serious Shocks with Ground Fault Interrupter" by Jerome Frank at pages 17-23 of the May 1973 issue of PLANT OPERATING MANAGEMENT. The basic principle of operation of a GFI is simple and well known. Any current from a load, e.g. a live appliance, tool or fixture, that passes through the human body to ground would not return via its normal path to the neutral grounded conductor. A GFI employs a differential transformer, normally of the toroidal construction, where the hot (live) and neutral conductors pass through the transformer. Should the current going to the load return through the neutral conductor in the normal manner, the two currents cancel each other out and no voltage is induced in the differential transformer. However, if a fault exists and current flows through a person to ground, this current does not return to ground through the differential transformer and the resulting imbalance or difference in current is sensed by the transformer. This induced voltage now is amplified to provide enough energy to trigger a solid state switching device, usually a silicon control rectifier (SCR). The SCR which now has been turned on by the amplified signal from the transformer supplies power, usually to a solenoid type device. The energized solenoid trips a mechanism to open the circuit. All this must take place in 1/40th of a second or faster.
U.L. requires that a GFI device incorporate a test button and a means of resetting the device should it trip out due to a fault. Usually, the test button connects a resistance which would create a 5-milliampere fault current from the hot line after the differential transformer to the neutral return line before the transformer, thus simulating a fault current. U.L. further requires that the device be trip-free. This means that should a person hold the reset button in activated position, the GFI device still must trip free should a fault current exist. This also means that the trip mechanism must be able to operate mechanically independent of the linkage employed by the reset mechanism. This same requirement exits for circuit breakers and it is because of this that GFI's presently on the market employ a mechanical circuit breaker type mechanism. Due to the fast acting time required by U.L., the initiating solenoid which releases the mechanical parts to open up the circuit must be very sensitive and tolerances on all moving parts become rather critical. With the passage of time, circuit breaker mechanisms lose their consistency and the operating characteristics are adversely affected. Parts wear, lubrication areas dry out, corrosion may occur between unlike materials in mechanical linkages, and other factors will cause changes in the operating characteristics in the GFI.
The GFI may become too sensitive and trip out because of wear or vibration. A far more serious weakness exists when the parts do not move freely enough and the GFI may not activate fast enough or not activate at all when a dangerous fault current exists. This is another reason why U.L. requires that each GFI be supplied with a chart to be mounted near the device where periodic tests are performed and recorded on this chart at least once a month to denote that the GFI is operating properly.
Present GFI devices on the market come in three basic forms.
The first is a standard type circuit breaker with a GFI circuitry built in. These circuit breakers have a supplementary solenoid to trip the mechanism which is operated by the GFI circuit. They are so designed that they would also trip out due to normal overload of current.
The second is a portable plug-in device which incorporates usually a GFI circuit breaker and a convenience outlet where portable tools or appliances can be plugged in.
The third is a standard type duplex wall receptacle with the GFI circuitry and interrupting mechanism built right in. The receptacle is normally installed as the first outlet after the circuit breaker or fuse in a branch circuit. If the device is of the feed-through type, other receptacles in the circuit beyond the GFI device are protected. Should a receptacle in the circuit become de-energized because the GFI tripped out, it then becomes necessary for the person using the tool or appliance to search around and locate the GFI device in order to reset the circuit.
Another disadvantage of all these devices lies in the fact that a second person could reset the GFI while the person shocked still is holding the faulty appliance. This would result in repeated shocks until the fault is corrected.